In exhaust system of automobiles, various catalysts for purifying exhaust gas, such as oxidizing catalysts, three-way catalysts and NOx storage-reduction catalysts are loaded. By these catalysts for purifying exhaust gas, harmful components in the exhaust gas are purified. In these catalysts for purifying exhaust gas, catalytic activity is developed by a noble metal, such as Pt (platinum), Rh (rhodium) or Pd (palladium). The noble metal is used under the loaded condition onto a support comprising a porous oxide, such as alumina.
In order to develop the maximum activity of the noble metal, it is preferable for the particle of the noble metal to be loaded under the high dispersion condition and as fine as possible. Therefore, the noble metals are loaded onto the support by an absorption loading method or an impregnate loading method using solution of soluble noble metal compounds, for example complex salt or nitrate. The noble metals loaded by these methods are loaded under high dispersion condition as fine particles having extreme small particle diameter. Thus, high catalytic activities can be developed.
However, there has been such a problem of catalyst deterioration that when the catalysts for purifying exhaust gas are exposed to high temperature at the time of using, the fine particles of the noble metals aggregate each other to make coarse particles resulted in the deterioration of the catalyst activities. This aggregation of the noble metal particles is easy to occur particularly in the oxidizing atmosphere. Therefore, particularly, it is problematic to the case of the lean burn engine which can restrain discharge of carbon dioxide. For these problems, the noble metals are loaded in larger quantity than that to be needed anticipating the deterioration. However, these methods need to be considered in view of the cost.
Accordingly, it is effective to regenerate a deteriorated catalyst for purifying exhaust gas. For example, Japanese Unexamined Patent Application Publication No. 2002-282,710 discloses a method of regenerating a catalyst for steam reforming reaction by supplying fuel and air to the catalyst. According to this method, the catalysts are heated to 500° C.˜800° C. by the combustion of fuel. The catalysts are regenerated by reducing the noble metals under the heated condition.
However, the above-described method in No. 2002-282,710 is inefficient because the regenerating process needs to interrupt supplying fuel and steam to the catalyst and carrying out the steam reforming reaction. Moreover, the drivability may deteriorate when this method is applied to the regeneration of a catalyst for purifying exhaust gas of an automobile.
Hence, Japanese Unexamined Patent Application Publication No. 2001-058,121 discloses a regenerating apparatus and a regenerating method for regenerating a catalyst for decomposing dioxin or the like. According to this technology, a deteriorated catalyst can be regenerated by processing with ammonia type reducing agent. However, according to this technology, the use of catalyst has to be interrupted because the catalyst is taken out for regeneration. Moreover, the used catalysts are Ti-V system catalysts and noble metals are not used in No. 2001-058121.
Moreover, Japanese Unexamined Patent Application Publication No. 2006-118,500 and Japanese Unexamined Patent Application Publication No. 2006-299,857 disclose a method for regenerating a catalyst in use. However, the former is a method for regenerating by reducing NOx storage-reduction catalysts with a hydrocarbon partial oxide and is not a method for regenerating purification activities of noble metals. The latter is a method for regenerating a diesel particulate filter. It is a method for regenerating a filter by removing deposited diesel particulates and is not a method for regenerating purification activities of noble metals.
Patent document No. 1: Japanese Unexamined Patent Application Publication No. 2002-282,710;
Patent document No. 2: Japanese Unexamined Patent Application Publication No. 2001-058,121;
Patent document No. 3: Japanese Unexamined Patent Application Publication No. 2006-118,500; and
Patent document No. 4: Japanese Unexamined Patent Application Publication No. 2006-299,857